Maddening isn't it?
You have to enter the field to scroll it, but this trips the trigger and then you can't....
What are you trying to achieve with this script trigger? (Maybe we can figure out an alternative).
I first encountered this little gotcha a while back when I figured out that I could set up a text field such that clicking text styled in a certain way could become a "hyperlink" of sorts using the OnObjectEnter script trigger and a script that takes the cursor position and parses the text with that style to the immediate left and right of the cursor in order to determine how to respond to the mouse click. I could find and open a graphic illustrating the text, for example using the "figure 1" text with an italics text style, for example, as the hyper text. This all worked, but only as long as I did not try to use a scroll bar with the edit field.